This application claims priority to an application entitled xe2x80x9cMethod for Controlling Heat Treatment in Process of Fabricating High Purity Silica Glassxe2x80x9d filed with the Korean Industrial Property Office on Dec. 30, 1999 and there duly assigned Ser. No. 1999-67040.
1. Field of the Invention
The present invention relates generally to a method for fabricating a silica glass and in particular, to a method for controlling the heat treatment used to remove impurities during the fabrication of a high purity silica glass.
2. Description of the Related Art
It is highly desirable to obtain optical communication systems with high speed transmission capabilities and low loss communications through optical fibers made from a preform composed of a silica glass.
The silica glass is fabricated by using a quartz raw material process, a synthetic quartz grain process, or a sol-gel process. Particularly, the silica glass fabrication method via the sol-gel process is disclosed in detail in U.S. Pat. No. 5,240,488, entitled xe2x80x9cManufacture of vitreous silica product via a sol-gel process using a polymer additivexe2x80x9d.
FIG. 1 is a flow chart illustrating the process of fabricating a silica glass via a sol-gel process according to the prior art. Referring to FIG. 1, the conventional method for fabricating the silica glass via the sol-gel process consists of a sol forming step 10, a molding step 20, a gel aging step 30, a de-molding step 40, a drying step 50, a low heat processing step 60, and a sintering step 70 in sequence. The process for fabricating a tubular silica glass, such as a substrate tube and an over-jacketing tube, will be described herein below.
The sol-gel forming step 10 is characterized by mixing starting materials with de-ionized water and an additive, such as a dispersant or the like, to form an uniform sol. The starting materials are generally selected from either a silicon alkoxide for forming a polymeric sol or a fumed silica for forming a colloidal sol.
Then, the molding step 20 is performed to inject the sol generated through the sol forming step 10 into a mold with a specific shape to form a gel. A binder for coupling particles or an additive, such as an accelerator for gel forming, is added to the sol.
Next, the gel aging step 30 is performed to remove a central rod from the mold and then performs the aging process for the molded composition.
Subsequently, the de-molding step 40 is performed to separate the aged gel from the mold. During the de-molding step 40, water pressure is used within a reservoir in order to protect the aged gel from damage.
Thereafter, the dry step 50 is performed to dry the aged gel separated from the mold in the de-molding step 40 in a humidity chamber by applying constant temperature in order to form a first dried gel. After the constant temperature and humidity drying, a second drying is performed under constant temperature and humidity.
Then, the low heat processing step 60 is performed to apply heat treatment on the dried gel with a gas (i.e., chlorine, hydrogen, oxygen, etc.) to decompose organic materials (i.e., residual moisture and binder within the dried gel) to remove impurities (i.e., metallic impurities and OH groups). The low heat processing step 60 is sometimes referred to as a purifying step as this type of process removes impurities within the dried gel as earlier.
Finally, the sintering step 70 is performed to sinter the dried tubular gel obtained during the low heat processing step 60 at a high temperature to produce a final product-namely, a silica glass. The sintering phase 70 applies heat up to 1450 degree C. to the purified and dried gel inside a sintering furnace, which is longitudinally movable while being exposed to He gas. After the sintering step 70, a high purity silica glass, such as a substrate tube or an over-jacketing tube, is finally obtained.
In particular, the low heat processing step 60 is typically performed under a low temperature heater with an inhalation line and an exhaust line. In order to obtain a low-heat-treated gel having a uniform distribution throughout the low heat processing step 60, the pressure within the low temperature heater should be maintained constant. This is because residual moisture, organic additives, metallic impurities, and any hydroxyl group within the dried gel is affected by the pressure within the low-temperature heater. Other factors that may influence the pressure within the heater include a mass flow of the process gas supplied into the inside of the low-temperature heater via the inhalation line, the pressure of the exhaust gas, the size of the exhaust line, etc.
During the low heat processing step 60, the size of the exhaust line needs to be frequently changed due to a component conversion in the conventional low heat process. The diameter of the exhaust line may be often subject to a certain degree of change depending upon the capacity of the heat treatment equipment or heater in use. For example, as the capacity of the heat treatment increases from 1 gel/cycle to 4 gels/cycle, the diameter of the exhaust line also needs to change from xc2xd inch to 1 inch. However, when the exhaust line changes, causing a change in the mass flow of the process gas, the process pressure within the low temperature heater is accordingly affected, thereby deteriorating the quality of the dried gel. For example, when the mass flow of the process gas is abruptly changed in the exhaust line to a larger diameter, the velocity of the exhaust gas will decrease, which in turn will change the process pressure within the low-temperature heater. Then, the result of change in the process pressure will disadvantageously deteriorate the quality of the dried gel undergoing the low heat processing. Therefore, there is a need to provide a mechanism to maintain the process pressure during the low heat treatment of the fabrication of a high-purity silica glass.
It is, therefore, an object of the present invention to provide a method for controlling the heat treatment during the fabrication process of a high-purity silica glass, so that a silica glass having a uniform distribution can be produced even when the size of an exhaust line of the low-temperature heater is changed abruptly.
Accordingly, the present invention provides a method for maintaining a constant gas pressure within a low-temperature heater with an inhalation line and an exhaust line during one of the sol-gel process used to fabricate a high-purity silica glass, the method comprising the steps of: detecting whether the diameter of the exhaust line is changed; selectively adjusting the flow of gas inputted to the inhalation line of the low temperature heater if the diameter of the exhaust line is changed; measuring an exhaust gas velocity discharged through the exhaust line; and, comparing the current exhaust gas velocity with a previous exhaust gas velocity before changing the diameter of the exhaust line.